Metal treating process

ABSTRACT

AN IMMERSION PHOSPHATING PROCESS FOR IRON AND STEEL WHEREIN THE STEEL IS CONTACTED AT ROOM TEMPERATURE TO 50* C. WITH A ZINC PHOSPHATE SOLUTION CONTAINING NITRITE AND AT LEAST ONE SOLUBLE REDUCING SULFUR-OXYGEN COMPOUND IN WHICH THE SULFUR HAS A POSITIVE VALENCE OF FROM 2 TO 5. SUITABLE COMPOUNDS INCLUDE NA2SO3, NA2S2O3, NA2S2O4 AND NA2S2O5.

United States Patent 3,671,332 METAL TREATING PROCESS Werner Rausch, Stierstadt, Taunus, and Hans Young Oei and Siegfried Moller, Frankfurt am Main, (fermany, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y. No Drawing. Filed July 15, 1969, Ser. No. 841,993 Int. Cl. C23f 7/10 US. Cl. 148-6.17 3 Claims ABSTRACT OF THE DISCLOSURE An immersion phosphating process for iron and steel wherein the steel is contacted at room temperature to 50 C. with a zinc phosphate solution containing nitrite and at least one soluble reducing sulfur-oxygen compound in which the sulfur has a positive valence of from 2 to 5. Suitable compounds include Na SO Na S O Na S O and Na2S205.

It has been known for a long time that iron and steel may be treated at room temperature and up to 50 degrees centigrade by immersion in nitrate containing phosphate solutions based upon zinc phosphate, whereby a strongly adhering zinc phosphate layer is produced upon the metal. The purpose of the nitrite, which is usually added to the bath in the form of alkali nitrite, is to accelerate the process of film formation and to convert the dissolved iron into its three-valent form to enable its precipitation from the bath in the form of the difiicult to dissolve iron- III-phosphate.

It has been found, however, that whenever there is a particularly large throughput of iron and steel being treated, the nitrite upon the iron surface is no longer in the position to completely oxidize the dissolved iron in a sufficiently short time. This condition may be recognized by an increasing brown discoloration of the bath, due to the formation of the well known ferro-nitroso complexes. The two valent iron bound in this complex remains stable even when greater quantities of dissolved nitrite are present in the bath and may remain practically unaffected over several hours. With increasing throughput quantity, finally a condition is reached wherein the bath solution is at no time free from ferro-nitroso compounds. The stationary concentration of ferro-nitroso compoundincreases with increasing throughput frequency under decreasing stationary nitrite concentration and decreasing bath temperature conditions. The ferro-nitroso-complex enrichment in the bath is undesirable on several grounds, and with the increasing iron content in the bath there is an increasing danger that coating films containing iron- II-phosphates will be produced. The anti-corrosion properties as well as the adhesion strength of the layer to the base metal will be considerably less effective, furthermore, as great quantities of nitrous gases are bound in the complexes which, for example under agitation of the bath, may be easily liberated and pass into the atmosphere of the workshop, thereby creating industrial health hazards. Such agitation of the bath may occur for example when introducing or removing the work pieces from the bath as well as during cleaning.

It has now been found that the undesirable formation of ferro-nitroso complexes in nitrite containing phosphatizing solutions based upon zinc phosphate, may be essentially reduced during the immersion phosphate coating process of iron and steel in the temperature range from room temperature to 50 degrees centigrade, by adding to the phosphate coating bath apart from nitrite and by way of additional components, one or more soluble, reducing,

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sulfur-oxygen compounds, wherein the sulfur is in positive form, having valency from 2 to -5.

The addition of the reducing sulfur-oxygen compound may be effected by means of gaseous S0 Na SO Na S O NflzSzOs, N21 S O N32S203, (Rongalite) and similar materials. The addition of the reducing sulfur-oxygen compound calculated as Net- 50 should be preferably 0.0l-l g./l. Preferably, per mol of N0 less than 1.5 mol of reducing sulfur-oxygen compound, calculated as SO should be used. The nitrite is being added to the phosphatizing solution preferably in the form of alkali nitrite, for example, NaNO This may be added to the bath preferably in quantities ranging from 0.1 to 1 g./l., calculated as NaNO Apart from zinc phosphate, other film-forming cations, for example, Mn, Ca, may be also contained in the phosphatizing baths prepared according to the method of the present invention. Apart from nitrite, other oxidizing agents such as for example nitrate may be also present in the bath. Copper and/or nickel additions, as well as the presence of simple and/or complex fluorides will accelerate the process of film formation. The basis weight of the phosphate layer may be reduced in a manner known per se, by the addition of condensed phosphates, hydrocarbon acids and similar materials. Wetting agents will promote the penetration of the bath solution into densely packed work pieces, such as for example wire coils.

The addition proposed according to the present invention is generally fully utilized only when the phosphatizing solution is used in conjunction with dipping or immersion processes. With spraying process, the movement or the agitation of the bath is so great that even with great throughput frequencies detrimental quantities of ferronitroso complexes usually do not accumulate. The preferred bath temperature is either at room temperature or slightly higher. Under these conditions the formation of ferro-nitroso complexes is being particularly promoted. This may be seen quite clearly from the example of various experiments, wherein steel wires were phosphatized by immersion into a 45 point zinc phosphate bath, containing Cu, N0 F and about 700 mg./l. NaNO At a throughput density of 0.2 m? per liter of bath solution and during 24 hours of operating time, a bath temperature of 30 degrees centigrade was suflicient to maintain it practically free from ferro-nitroso complexes through the replenishment of the NaNO content to maintain it at 700 mg./l. On the other hand, when the throughput frequency was increased to 0.6 mF/l. in 24 hours, at 30 degrees centigrade, after a very short time a pronounced brown discoloration ofthe solution was observed. In order to avoid the disturbing formation of ferro-nitroso complexes, under the same conditions the bath temperature had to be raised to 55 degrees centigrade, which in turn brought about an undesirable increase of heating costs.

The addition according to the present invention may be used for new baths as well as for baths already in operation. In the first instance, the bath is prepared in the customary manner with zinc phosphate concentrate and, if desired, by the necessary further additions, and then, before the start of the throughput of work pieces, the bath is reacted with about 300 to 600 mg./l. NaNO and with about an identical quantity of reducing sulfur-oxygen compound, for example Na SO During the operation of the bath, the zinc phosphate nitrite and sulfur-oxygen compound are being continually consumed. The replenishment of the zinc phosphate may be effected, for example in the customary manner to the consistency of the points of the bath by means of a replenishment concentrate. The nitrite may be added to the bath, for example in such quantities that the reducing effect of a sample taken from the bath remains approximately constant when determined against KMnO -titration solution. When in the course of this replenishment and after a few throughputs the bath still remains brown colored for a longer time, i.e., for about 10 to 20 minutes after the removal of the last charged, then again the sulfur-oxygen compound should be added in approximately the same quantity as initially. After this addition and after the resumption of the phosphorizing process, the solution will become either clear again or will discolor only to such small extent that at the latest minutes after the removal of the last charge the solution will become clear. Through this procedure it is easy to determine the time when it becomes necessary again to add sulfur-oxygen compound to the bath. It is further possible to determine the quantities of consumed NaNO in the course of the operations as well as the quantities of the sulfur-oxygen compound required from time to time, and to prepare such a replenishing salt which contains both components in the determined weight proportions. When such a mixture is being used for replenishment and for the maintenance of the constant replenishing effect of the bath as determined against KMnO then the bath will remain practically free from ferro-nitroso compounds even under high throughput frequency conditions.

When applying this process to the baths currently in production and which already contains greater quantities of ferro-nitroso compounds, then, for example, the following procedure may be followed. It should be established first whether the bath contains sufficient nitrite quantities in order to oxidize all Fe II to Fe 111. If it is necessary, then nitrite should be added, and subsequently about 300 to 600 mg./l. sulfur-oxygen compound is added to the bath, and this process is repeated at about every quarter of an hour until the color of the bath becomes light. After this, operation may be resumed in the manner described in the foregoing.

The coatings produced according to the method of the present invention are suitable for the known applications of phosphate coatings, such as anti-corrosion, to promote noncutting cold shaping, electrical insulation, reducing friction, and the like.

In order that those skilled in the art may better understand the method of the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, parts and percents are by Weight and temperatures are in degrees centigrade.

EXAMPLE 1 The roll scales were removed from steel wire sections in sulfuric acid, then these wire segments were rinsed in Water and subsequently phosphatized by dipping at 30 degrees centigrade. The phosphatizing bath was an aqueous solution having the following basic composition:

The bath was adjusted with NaOH to a free acid content of about 4.3 points and to a total content of about 40 points. Subsequently, 0.1, 0.2, 0.4, respectively 0.6 g./l. Na SO quantities were added to this basic bath. Into the individual baths, in every instance 400 cm. steel surfaces were dipped for minutes per 1 liter of phosphatizing solution. Immediately after the removal of the wires, respectively after 10 minutes, the Fe II contents of the baths were determined by colorimetry with ortho-phenaflthroline. Furthermore, a visual assessment was used to establish the time period required for the baths to regain their clear color. The results are summarized in Table I.

TAB LE I [Fe II (g./l.)]

Irnrnedi- Time required ately after 10 min. for bath to hosafter phosbecome clear Phosphatizing bath phatizing phatizing (min) Without addition 0. 05 0. 03 Plus:

0.1 g./1. N21280:; 0. 01 0. 0O 12 0. 00 0. 00 5 0. 00 0.00 2 0. 00 1 No difierences were found between the phosphate coatings and uniform coverage was obtained.

EXAMPLE 2 Steel wire sections were pretreated in the manner described in Example 1, and subsequently phosphatized by dipping for 10 minutes at 30 degrees centigrade. In experiment A, a phosphatizing bath was used which corresponded to the basic composition described in Example 1, including the corresponding adjustment of free acids and total acidity. In experiment .B a bath of identical composition was used, however, with the addition of 0.6 g./l. Na SO at the start.

The replenishment of the baths was carried out with a replenishing concentrate having the following composition:

Percent Zn 12.2 P 0 22.2 Ni 0.02 Cu 0.20 F 0.70 N0 8.2

based upon the stability of the total acids, While the NaNO quantity was determined on the basis of the stability of the reducing effect of the solution against KMnO The additional KMnO consumption in iron-II- containing baths was taken into particular consideration.

In Experiment B during the charge throughput in each instance when brown discoloration was observed, further 0.3 g./l. Na SO was added. The throughput frequency in both instances was 0.8 m./l. in 24 hours. The test results are summarized in Table II.

EXAMPLE 3 A phosphatizing solution, having a basic composition according to that described in Example 1 was used. Per every 1 liter of this bath, 800 cm. of steel plate surface was dipped for 20 minutes at 30 degrees centigrade, and this has resulted in dark brown discoloration. Afterwards determined quantities of various reducing sulfur-oxygen compounds were added to the baths. The time required for the discoloration to disappear were determined after 2 hours of standing period of 30 degrees centigrade. The results are summarized in Table III.

TABLE III Time required Oxidation for complete number discolora- Color after Addition of S tion (min) 2 hrs.

Without 120 Dark brown. N 212 203 0.1 g./l 120 Light brown.

0.5 g./l 90 Colorless. N32820::

0.1 120 Light brown.

0.5 90 Colorless. NflzSzO 2 0.1 120 Light brown.

0.5 90 Colorless.

What is claimed is:

1. A method for treating ferrous metal surfaces which comprises immersing the surfaces to be treated in an aqueous zinc phosphate phosphatizing bath maintained at a temperature from about room temperature to about 50 C., which bath contains nitrite ions in an amount from about 0.1 to 1.0 gram per liter, calculated as NaNO and at least one soluble reducing sulfur-oxygen compound independent selected from a group consisting of S 13.2803, Na S O Na S O Nazsgoi Na S O and NaHSO CH O, wherein the sulfur-oxygen compounds in the bath are present in an amount from about 0.01 to References Cited UNITED STATES PATENTS 2,121,520 6/1938 Curtin 1486.17 2,591,479 4/1952 Ward 148-6.15 X 2,762,733 9/1956 Borghetti et a1. 148-6.17 2,975,082 3/ 1961' Henricks.

FOREIGN PATENTS 741,937 11/1943 Germany 1486.15

OTHER REFERENCES Translation of German Pat. No. 741,937, pp. 7 and 8.

RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R. 1486.15 Z 

